texts: Orbital Angular Momentum of a Binary Star System

texts:
Let's say you have a binary star system, consider the orbital motion of that binary star consisting of two solar-like stars separated by 20 stellar radii. How would one calculate the angular momentum for this system?
The angular momentum we want is defined by the equation:
$L = I\Omega = Mk^2 R^2 \Omega$
Where $M$ is the Mass, $R$ is the radii of the orbit, $k$ the gyradius, and $\Omega$ the angular velocity.
$M$ and $R$ are easy enough. I'm confused on how to get $k$ and $\Omega$? Would the gyradius $k$ be what was given to me in an earlier part of the question (0.25) for when I had to calculate the Angular momentum of the Sun or would I have to calculate it separately for this case? What about $\Omega$? Would it simply be $\frac{2\pi}{28 days \cdot 24 hours \cdot 60 mins \cdot 60 secs}$ where the period of rotation comes from the Sun?
Clarification/help is appreciated. Thanks


texts: How to find which part of the sky a photo contains?

texts:
I was taking some photo stacks yesterday, and I also took a plenty of random ones (pointed not at a specific object) too at 200mm.

However, I've ended with a shot but I have no idea where I pointed my camera at, and because of the narrow angle of 200mm, I'm lost without any guiding star/object.

This happened to me before too, so I was wondering whether there is a software or a website that I can upload a photo, and it will analyze the objects, cross match them with known objects and detect which part of the sky it is? Or is there a method that I can use to figure out by myself (remember that there are no well-known bright stars in the image, at least not that I can identify by myself)?


texts: Could black holes be creators of dark matter?

texts:
At the tremendous forces of gravity inside a black hole, can "matter" be transformed into a more  degenerate form; "dark matter" (if that truly what dark matter is)?

I know the inside of a black hole isn't fully understood, but can this be an acceptable possibility? Why?


texts: Is there a way to factor age into the mass-luminosity relationship for stars?

texts:
I'm wondering if there's a way to formulate the luminosity of a star not only as a function of mass, but of age, too, and if so, what the formula for luminosity would look like. In the case of the Sun I'm aware it gets 10% brighter every billion or so years, but I doubt this relationship holds true for all mass ranges of stars, just like you have to adjust the formula for the luminosity mass relationship for stars of different masses.

If anyone could "shed some light" on this matter, it would be very much appreciated.


texts:
In this youtube video, Prof. Michio Kaku mentions there could be multiple universes and if there are, then there could be a worm hole to some other universe.

My question is: We are not sure if the Multiverse exists, so how can we believe or have imagined that worm holes could exist and create a path to another universe?


texts:
Saturn was formed around billion years ago along with Jupiter. After which it had enough time to form moon which can break into rings.

Then why is it said that Saturn's ring are of recent origin - about few million years ago?


texts: Will black holes evaporate, if they evaporate? When and why do they evaporate? What are the conditions for evaporation? Under what principle do they evaporate?


texts: The cores of gas giants are kept under incredible pressure by the weight of the gasses of the giant. In a chthonian planet, said gases are stripped away, revealing the core of the gas giant, which should alleviate the pressure compressing the core of the gas giant. In a chthonian planet candidate, Kepler 52-c, its density is over 600 times that of Jupiter. What is keeping the planet at such a high density?


texts: What is the most dense object in the universe?

texts: Inspired by this answer to the question, Why is the Sun's density less than the inner planets?, what is the most dense object in the universe?


texts: 76mm reflector vs 70mm refractor? Both are at sale at $200

texts:
I'm starting my astronomy hobby and looking for a beginner telescopes to start with. I've searched couple websites and found these 2 within my budget.

I know for a fact that refractors gives you sharper and clearer images but its original price is actually cheaper than the reflector telescope. I will list the specifications below. It would be really appreciated if you guys can tell me the relative benefits and drawbacks of each.

Refractor: Saxon 70mm Telescope

OPTICAL DESIGN: Refractor

APERTURE: 70mm

LOWEST PRACTICAL POWER: No

HIGHEST PRACTICAL POWER: 165x

FOCAL LENGTH: 400mm

FOCAL RATIO: F/10.0

EYEPIECES: 20mm & 10mm

FINDERSCOPE: 5x24

DIAGONAL: 1.25" 45° Erect Image Diagonal

MOUNT TYPE: Alt-Azimuth

TRIPOD: Aluminium

This telescope includes a pre-assembled tripod, two 1.25” eyepieces, a cleaning cloth and a bonus padded backpack.

Reflector: Saxon 767AZ Reflector Telescope

OPTICAL DESIGN:Newtonian Reflector

APERTURE: 76mm

LOWEST PRACTICAL POWER: No

HIGHEST PRACTICAL POWER: 152x

FOCAL LENGTH: 700mm

FOCAL RATIO: F/9.2

EYEPIECES: H25, H12.5 and SR4

FINDER-SCOPE: 5x24

BARLOW LENS: 3x

DIAGONAL: No

MOUNT TYPE: Alt-Azimuth

TRIPOD: Aluminium

OPTICAL TUBE DIMENSIONS: 12.5 cm x 68.5cm

OPTICAL TUBE WEIGHT: 1.76kg

SHIPPING DIMENSIONS: 74 x 25 x 23cm3

SHIPPING WEIGHT: 5.4kg

The accessory pack includes three 1.25" eyepieces, a finder-scope, an erecting eyepiece for terrestrial viewing, an adjustable aluminium tripod with accessory tray and a moon filter, which reduces the brightness of the moon and makes surface details sharper and clearer.


texts: Why has there been no moon for 2 nights straight

texts: I have been watching the sky all night for a week now I see flying stars everywhere well things that look like stars, what are they? And the moon has been gone for two nights now? Why?


texts: From where so much of energy would have arrived as was required for such a giant universe to have come into existence?

texts: If we convert the mass of all the stars and all the planets which must be weighing trillions and trillions of tonnes by the equation E = m.c^2 it gets converted into an unimaginable amount of energy. Would so much of energy not have gone into the evolution of the Universe? I am at a great suspense whether the Big Bang theory or any other theory at all answers from where so much of energy would have come that would have got converted into so many trillions and trillions of tonnes of the mass of all the galaxies put together. How we can believe in the big-bang-theory unless we know from where so much of energy would have arrived?


texts: If Kepler-444 planets existed for 11.2 billion years, why fear for life on Earth after six billion years?

texts: According to recent news, Kepler-444, a 11.2 billion year old star, has a system of five planets. It has a mass of 3/4 of solar mass. So I believe both Kepler 444 and the Sun will share the fate of 'orange main sequence stars'. If so, while scientists fear for life on Earth in the coming billion years, which is only one sixth for our solar system lifetime, how is it that the Kepler-444 planets have existed for a longer period of time?


texts:
Good day to everyone.
I wanted to know what is the importance of estimation of line flux ratios in active galaxies? What does it help in characterizing?
As in Lyman $\alpha /$Carbon IV, or Mg II$/$Carbon IV, or Carbon III$/$Carbon IV
Do you have any references about this? Kindly do share.


texts: Assuming 2007 OR10 is in tidal lock with its satellite, what is its mass?

texts: Its rotation period is 45 hours. Its satellite is at the distance of 15000 km. Given this data, and assuming the satellite's orbital period the same as the main body's rotation period, 45 hours, what is the mass of the main body?


texts:
The History of Science and Mathematics stack exchange has a question "When Did Telescopes become Affordable?" asking when telescopes became available to members of the general public instead of wealthy people, professional astronomers, sea captains, and army officers.
And I think that telescopes have become more affordable to the general public over centuries.
So do any amateur astronomers at this site have any information about the history of making telescopes to sell to amateur astronomers?


texts: Is the Milky Way Visible from Nebraska?  If so, where is the best place to view it, and also what would be the best time of night to see it?  I know this is probably a very novice question, just trying to get a better understanding of what I can see.  If it helps, I'm south of Omaha.


texts:
Is it posssible for us to time travel?
Is light essential factor or the dark matter and dark energy?


texts: Calculate how long it takes for the sun to complete one orbit around the galaxy?

texts: We know the speed that the sun travels around the galaxy is $220\, {\rm km/s} = 0.00021 \, {\rm parsec/year}$. I'm unsure of how to find the period?


texts: How bright would Alpha Centauri appear (in apparent Magnitude) if it were 1 light year away?

texts: I'm trying to determine what a star of comparable mass and luminosity to the Sun would look like from the surface of the Earth if it were 1 light year away - I chose Alpha Centauri since it is close to us, and similar to the Sun. Could a star like the Sun be that close without exerting significant influence over it, or would they have an orbital relationship? I'm trying to design a star system for a science fiction novel, and would like there to be a star close enough and bright enough to be visible during the day, and cast a shadow at night, but far enough away to provide no heat energy, and be free from the Sun's gravity.


texts: Why do we use FITS format for scientific images especially in astronomy? How is it different from formats such as JPEG, PNG etc?

texts: I am new to astronomy and I found that the telescopic dataset can be stored in the FITS format instead of JPEG. Why do we have a special image format just for scientific images?


texts: A question about stellar magnitude and the pogson's equation

texts:
Well surely the eye has different sensitivity to various wavelengths... will the pogson's equation  not be affected if say I compare Betelgeuse and Rigel( both having a marked difference in spectral lines)...

also, how can we ever draw a standard in stellarnmagnitude observed if both location and individual visual acuity?

does atmosphere also have any effect??

Please refer some good resources for further study in this topic.


texts:
What is a Schmidt-Cassegrain (SC) telescope?

We have Newtonian telescopes and also Galilean ones. How is SC telescope different from other optical telescopes?
What is its construction detail and what different mechanism does it work with?


texts: Do planets have effect on velocity of Earth?

texts:
There is method for finding extrasolar planets called Transit-timing variation. If I understood correctly, exoplanets accelerate or decelerate each other (according on their position), so we can observe irregularities in transit times of some exoplanet.
Now, for our solar system, can we observe similar effect here? Why time of Earth's orbit around Sun is always 365.256 days, even though other planets are always in different places? Shouldn't Earth year be always differently long?


texts: Why is the E-mode polarization spectrum out of phase with the Temperature spectrum?

texts:
The E-mode polarization power spectrum of the Cosmic Microwave Background displays the same acoustic peaks that can be seen in the (more famed) temperature power spectrum. However, they are out of phase with respect to each other. A number of sources give answers that all come down to the following:

  The main EE' power spectrum has peaks that are out of phase with those in theTT' spectrum, because the polarization anisotropies are sourced by the fluid velocity.

But I fail to understand this. Can someone elaborate on this?


texts: Why does the focal length of a telescope have an effect on the magnification?

texts:
I previously asked that

  Is the angular resolution of a telescope irrespective of used eye-piece?

I learned that the resolution is fixed when the light enters the telescope, and that the eye-piece is used just for magnifying the obtained image at the focal plane.

Now, the magnification of a telescope is calculated as

$$
\text{magnification} = \frac{\text{focal length of telescope}}{\text{focal length of eye-piece}} \qquad (1)
$$

For example, if the focal length of my telescope is $900$ mm and I am using a $25$ mm eye-piece, the magnification I get is

$$
\text{magnification} = \frac{900\ \text{mm}}{25\ \text{mm}} = 36.
$$

However, where does the formula $(1)$ come from? Why is the magnification affected by the focal length of the telescope and eye piece?

(I am using a Newtonian telescope if it matters.)


texts: Must all galaxies in a galaxy cluster have the same expansion rate?

texts: If we observe a distant galaxies cluster where all galaxies travel with the same speed away from us, except for one galaxy whose speed is significantly lower than of the neighboring galaxies. Does this necessarily contradict the principle of universe expansion?


texts: How would a small nearby black hole be detected and confirmed as such?

texts:
This article suggests that there might be a tennis ball-sized black hole orbiting the Sun somewhere beyond the orbit of Neptune. According to the article, observational anomalies of trans-Neptunian objects suggest that there is an object out there about 10 times the size of Earth (I assume it to mean about 10 Earth mass). Since searches have failed to find such an object, an alternative hypothesis claims that there may be a primordial miniature black hole out there. The article goes on to state that a black hole has not be found there because searches have been for planets and the methods used to find planets are different than those used to find black holes.

My question is: how would such an object (mini black hole) be detected, to either confirm or rule out its existence?


texts: Has the Hubble ever been pointed at earth?

texts: As in taking pix of earth, not just happen to point towards earth during a maneuver or whatever.


texts:
Recently I've explored an area on Mars' surface which one looks like huge hatch cap & hole, about 10 kilometers, between tons of circular craters.

The formation is very unique and strange to me. Somehow it has moved from original location and broke into two pieces then half part disappeared.

What do you think about it?

Google Maps link(25.3717187,166.6337854)


texts: Could someone please explain to me how I can extract velocity component values from Stellarium, for a given planet?

texts:
For a project that I am currently working on, I would like to know how one could extract velocity component values ($v_x$, $v_y$ and $v_z$) for a planet from the Stellarium software. The current information panel for a planet does show its orbital velocity (which is the resultant velocity, $v$) but does not give the individual $v_x$, $v_y$ and $v_z$ values that result in that velocity.

What I essentially need, is to establish a velocity heading for the planet.

Any help (code-based/mathematical) would be much appreciated! Thanks in advance!


texts: Can we track matter through time by looking at different depths in space?

texts:
If we look back far enough we can see all the origins of the universe, so is it possible, even if not feasible, that we could trace the history of some matter as it moves through space-time? I want to understand how looking at different depths in space and time are correlated in regards to the matter being observed.

For example would it be possible to look deep into a certain part of space and time to find some galaxy that contributed to the matter that makes up the Milky Way today? Then somehow follow it through space-time by looking at different depths and locations in space, and see how it came to be part of the Milky Way?


texts:
Let's assume there is a right rectangular prism floating about in space, what would the gravity be like across its 6 flat surfaces? What gravity would you experience if you set foot on the rectangular prism's surface? Would it be the same as on Earth? Or would it be different the further along the surface you walk?

Let's assume that this right rectangular prism has a constant density and has a mass equal to Earth's. At any point on the surface of this prism, would gravity vary? That is, would it be an extreme variation than compared to one traveling across the surface of Earth? How extreme would this differential be?

Here's a rectangular prism with specific dimensions in mind.


texts:
According to 2nd law of thermodynamic,  in an isolated system entropy increases over tume. In an open system entropy might seems decreases but combined result of all related events results in overall entropy increase.

Now lets say, a suffled deck of playing cards. If I sort the cards it seems entropy decreases, what's then the related  events that increases overall entropy. I am asking just to clear some of my confusions.


texts: Does the "photo of a black hole" taken by EHT actually depict a visible light-based scenario?

texts:
I'm still at my bachelor of electronic and communications engineering, so I'm not an expert of radiotelescopes; however, I surely know that, typically, radio communications are characterized by a RX that has at least one bandpass filter.

As a result, I am led to think that the "photo" of the black hole, recently released to the public, is a reconstruction picture taken from various radiofrequency signal samples that got mapped to the frequencies of visible light spectrum (since the Event Horizon Telescope project is based on radio telescopes), hence it doesn't depict what we would actually see (neglecting any impossibilities in doing so), but rather, it shows the radio emission from whatever is close to the event horizon of the black hole.

Is this correct, or is it actually a photo capturing visible light?
I mean, scientifically, this is a huge step; however, I have the feeling that the news got "boosted" a little too much, in medias, as they just talk about "photos" and "light" surrounding a black hole - not specifying anything else for a inexperienced audience.


texts: What are the craters on the moon with peaks in the middle of them?

texts:
Was taking some extremely amatuer pictures of the moon from my cheap telescope with my camera phone...

You see the crater Im talking about? Top right.The one with a little nub in the middle- which Im sure is actually massive. How was this formed differently than the other craters? What couldve caused the hill?


texts: Projecting an image of the sun / eclipse

texts:
This might be an astronomy.stackexchange.com question, but I think it is more in line with a camera obscura.

During the last eclipse I didn't plan ahead and grabbed some lousy toy "spy" telescope we had (about 2 inches long), set it up on a tripod and pointed it at a white foam board.

I'd like to plan ahead more this time.

What can I use to project a decent image of the sun onto a foam board?  What kind of lens do I need?  Would a magnifying lens work?  How about a close-up diopter filter?  How about a 50mm lens?  (I'd like to use something I already have, or is cheap -- and I don't want to use a pin hole.)

My first thought was that a magnifying glass only focuses the sun down to a pin point (and would burn the board), but I'm recalling now that past that focal point you can project an image.


texts: Can we change Earth's orbit with modern technology?

texts:
It is well known that, according to Newton's 3rd law, spacecrafts entering or leaving Earth provoke tiny (insignificant) changes to Earth's orbit (see related question here).
My question is about significant changes. For instance, say we would like to shift the Earth's orbit closer to that of Mars (forget about the ecological consequences of this). Do we currently have the technology capable of doing this? For instance, something like a huge nuclear explosion? This question only deals with "natural", non-human ways to change a planet's orbit. I am interested about technology-based changes.


texts:
I am Abdul Kadhir from Kochi, Kerala, India. I, my wife and my daughter have seen today (2020-01-11) morning 5.45 AM IST a line of stars. A pair of more than 30 stars moving in a line from south to north. In between one pair stars, there was a little distance and in between 2 pair the distance is 3 times. Roughly so: * *      * *     * *.

It was a wonderful experience. Single stars of about 20 came with in the same line with a little time gap... I want to ask, is this normally happening in the sky, or what is going on?

Every morning, when I wake up, I will look up the sky and after watching.


texts:
I just learned about precession of the equinox, i.e. the Earth's axis tilts one degree every 72 years.

And it takes 72 years/degree x 360 degrees =  25,920 (years in one cycle)

There are many theories to this, i.e.

Sun's gravity is affecting the tilt of Earth's axis
Sun itself is revolving around another star, and it takes 25,920 years, hence the slow tilt of Earth's axis

I googled whether the Sun revolves around something else, and according to NASA.gov, Sun revolves around center of Milky Way it it takes 230 million years.

Now, I am confused between precession of the equinox and Sun revolving around another star.

Does precession of the equinox have anything to do with the Sun revolving around another star? Or is it simply the Sun's gravity acting on the Earth?

What is the explanation according to modern science? I understand there were astronomers in the past who deduced that the Sun revolved around another star and it took 25,920 years.


texts: Why was there a gap in the number of asteroid detections between 1807 and 1845?

texts:
The first asteroid, Ceres, was discovered in 1801, although it wasn't called an asteroid yet.  Pallas, Juno and Vesta were discovered shortly after. Then no new asteroids were discovered for 38 years, until Astraea in 1845 when the rate of discovery picked up again, with the 10th asteroid (Hygiea) discovered in 1849.
Why was there such a decrease in new discoveries after the first four asteroids were discovered? And why did it pick up again a few decades later?
Was there a technological advance in the 1840s that made it possible to observe asteroids that were un-observable in the 1810s? Was the drop due to a decrease of interest in the astronomical community for objects between Mars and Jupiter? Did astronomers think they had found them all and stopped searching?


texts:
I recently read this article: http://www.cbsnews.com/news/supermoon-lunar-eclipse-coming-this-month/ I'm having some trouble understanding how a "supermoon" lunar eclipse is possible.

Judging from this diagram, it would seem that the moon always reaches its apsides above or below the orbital plane of the earth around the sun, making any kind of eclipse at these points impossible. Can someone clarify for me?

Thank you.


texts:
Since the direction of Vernal Equinox lies in the Pisces constellation, why is its symbol Aries? Shouldn't it be ♓ ?

Is this a historical anachronism, only invalidated by axial precession of Earth?


texts: Why we define Stellar motions with respect to sun?

texts:
We take observations from earth but to define radial velocity, transverse velocity and the proper motion of stars, why we consider them with respect to the sun? and then we do some corrections due to both rotational and orbital motion of the earth.

Why can't we directly consider these motions with respect to earth instead of Sun?

And what is the whole procedure of correction?


texts: Are there other planetary systems where gas giants are on the inside to rocky planet orbits?

texts:
I understand that formation theories for gas giants suggest they should be born further away where there is more gas for them to monopolize vs the sun, and then to form Hot Jupiters they need to migrate inwards, which would disrupt the orbits of rocky planets.

Are there any quibbles with this? Out of all the extrasolar systems is there even one where you have hot or warm gas giants and then cold Earth or Super-Earth sized terrestrials? Bodies like we have in the Kuiper belt lie beyond our gas giants but they are mostly made of ices and represent a small leftover fraction of the system mass, so they don't really count here.


texts: How should one rationally deal with the issue of space travelling alien civilizations?

texts:
What kind of reasoning is appropriate to understand the as of today unanswered question of whether there are (other) interstellar space travelling civilizations in the Milky Way?

We have already sent probes towards the border of the Solar system. And even landed human beings on another celestial body and brought them home alive and well. If we extrapolate the 50 years of space travel, the 100 years of electronics (radio), the 400 years of physical science, to just a fraction of the biological age of humankind into the future (like a few thousands of years), interstellar travel is not out of the question for us or at least our artefacts. So I imagine two possible alternatives:

1) The Milky Way is cluttered by lots of space travelling civilizations like us and our future. Once one of them/us gets going, they'll soon be everywhere. The Sun orbits the Milky Way every 250 million years, about 2% of the age of the galaxy. Going to the nearest stars is enough to soon be everywhere. But if they are everywhere since almost always, they should be here, we should be their seed.

2) We are the only space travelling civilization in the entire galaxy, ever. But then what makes us unique? We consist of the most common elements and volatiles of the universe and our planet and star and galactic location all seem to be very typical. There's no known trace of any uniqueness here. Whatever could it be?

Are there more alternatives?

While we cannot say today which alternative is true, we should be able to at least specify the possible alternatives. But to me they all seem to be absurd! What would be a rational logical scientific approach to this apparent paradox?


texts: I've seen a number of news reports indicating there is likely a 9th planet in our Solar System, something with an orbital period of between 10k-20k years, that is 10 times Earth's mass. I haven't seen any real indication of where this object might be. If I had access to a sufficient telescope, would I be able to find this planet, and what way would I point a telescope to find it? How far is it likely to be, or is that not well known?


texts: Corresponding values for the Sun's ultra violet color maps?

texts:
In my laboratory, we have an antenna to get data from the GOES-16 satellite and I want to reproduce NASA's nice visualization of ultraviolet images from the sensor SUVI.

I can read the data without any problem and thanks to the project sunpy I got the color tables corresponding to each band. But I have not found a reference for the exact correspondence between values and color indexes, so my version is not remotely as nice.

Where can I find a reference to know how to use this color map with my own software?  I will appreciate any hint.


texts: If there was air between the Sun and Earth, how warm would we get?

texts:
Based on this question: How loud would the Sun be?

If instead of a vacuum we had air between the sun and earth (let's say of the same composition we have here on earth) how warm would our planet get?


texts: If the Sun were bigger but colder, Earth would be hotter or colder?

texts: That is the question. I know the concepts of luminosity $(L=4\pi R^2F)$ and the flow $F=\sigma· T^4$, with $T$ the temperature in its surface. But how I use that to know if the Earth would get warmer or not if $R$ increases and $T$ decreases?


texts: As the Sun dies, does the Earth's orbit change?

texts:
As the Sun nears the end of its life and expands, would the Earths orbit get bigger as it moves out or would it stay in the same orbital plane and not move out?  Therefore in the far off future, we'd have 512 days instead of 365 days?

As the Moon is moving out at a distance of 3.8 cms a year (ref: http://curious.astro.cornell.edu/about-us/37-our-solar-system/the-moon/the-moon-and-the-earth/111-is-the-moon-moving-away-from-the-earth-when-was-this-discovered-intermediate), are we moving out or in towards the Sun or is this not possible to work out because there is nothing to measure against?


texts: What is the angular diameter of Earth as seen from the Moon?

texts:
...today I must have left my brain home. I know it's a simple calculation but I keep failing simplest calculations today.

What is the angular diameter of Earth as seen from the surface of the Moon?


texts: How much magnification is needed to see the planets of solar system?

texts: I have a 3inch Newtonian reflector telescope with 300 mm focal length. I can use highest magnification of 75x using a 4mm eyepiece. But in 75x I can't see the details of Jupiter what was expected. Instead I see a little blurry image. Now I would like to know how much magnification is necessary to see a good details of Jupiter and other planets. And one more question: Is there any way to improve the vision of my 3 inch telescope?


texts: stellar evolution fortran code-runge kutta mass step from center to fitting point

texts: I am trying to integrate the four stellar model equations from the core to a fitting point and from the surface to the fitting point ($x_f$). When I start the outward integration, however, I have to use a huge mass shell step $dx$ where $x_1=dx$ too and the callling routine has the form $rk4(dx,x,y,dy)$(Runge-Kutta 4th order with mass step $dx,x$ core). As a result I am getting a bus error. I do not know what is the appropriate step since if we consider that I want to integrate from $x_1$ to $x_f=10^{33}$, it means that I need a huge step $dx$ to make it work. I have tried it in many different ways and I cannot figure out how to properly do that and get a result when dealing with such huge numbers. If you have any ideas I would appreciate it, since I am stuck! Thank you very much!


texts: How fast can and do gravitational waves travel?

texts:
I was thrilled by the recent announcement of the detection of gravitational waves. The media has done a great job explaining them and the theory behind them and I get the concept fairly well.

But if it's a warping of spacetime, not a particle phenomenon,  (or is it somehow?) How fast do these waves travel through space? Is it  constant or variable? Slower than the speed of light or even faster? If spacetime has no resistance per se, the what would keep them from traveling infinitely fast?

I'd love to understand it better and hope my question makes sense.


texts: Where does Jupiter's gravitational force come from? Why don't Jupiters gasses fly away?

texts:
Gravitational force is the attractive force between any two masses.

What about gaseous planets, where do they get their attractive force from?

While the big rocky planets don't have enough attractive force to keep enough gases in their gravitational field, Jupiter has lots of gases.

Does this means there is a concentrated mass inside, under which gravitational field the gases are kept 'in'? And if so, then why are gaseous planets not also considered to be rocky ones?


texts: Is a comet necessarily confined to the solar system?

texts:
I had the following question:

  A comet enters the solar system. Assume that it is influenced only by the Sun, not by any of the planets. Which statement about its orbit is correct?

And I chose as correct statement:

  Its orbit might be an ellipse, or is might be a parabola or a hyperbola (because a comet is not necessarily confined to the solar system forever).

I want to know what it means by a comet is not necessarily confined to the solar system forever.

Does it mean that eventually the comet will "die" out, with the sun vaporizing all of its ice, so it will basically cease to exist while it is in the solar system?

Or does it mean that the comet can leave the solar system? If the comet enters the solar system it is subject to the mighty force of the suns gravity, then how can it escape?


texts: what is the highest throughput astronomy project in pixels?

texts:
I was having a discussing about big scientific projects with colleagues, and I postulated that astronomy is still one of the scientific fields with biggest projects per amount of data dealt with, after the high-energy physics.

I would like to know what is the highest throughput astronomy project as defined by the number of pixels in the images analysed in the project.

Something like "project XYZ" with an estimated "1.2 trillion pixels" worth of pictures taked and analysed over 5 years.


texts: Have any co-orbital exoplanet pairs been discovered (and not subsequently retracted)?

texts:
For this question, I think a good working definition of co-orbital configuration would be two bodies orbiting around a third much larger body in a 1:1 resonance and where neither mass is negligible. In other words, you can't really say that either one is at the other's $L_3$ because they are interacting with each other more as equals.

I found this article in the New Scientist in my first on-line search, but it has a retraction at the top.

Just for example, here is a theoretical analysis of stability in an ArXiv preprint on the subject: Dynamics of two planets in co-orbital motion.

With the caveats above (which I think are fairly standard for the term "co-orbital planet") have any been discovered?

edit: Have any new candidates been at least identified or proposed?


texts:
It is a well known fact that in stars, there exists a temperature gradiënt. The
observational reason is because we perceived spectral lines in the otherwise continuous spectrum of a star. If this wasn't the case, and thus if the temperature were uniform, then the absorption and the emission would cancel each other out and we would observe a net effect that results in a continuous spectrum. Now my question is: what is the physical explanation for this temperature gradient?


texts: Can a high enough density ignite a gas giant into a brown dwarf?

texts: I wonder whether the nuclear fusion of brown dwarfs and more massive stars really depends on their mass only or if it could also start nuclear fusion if it is dense enough but not as massive as brown dwarfs usually are (they need more than 12 Jupiter masses to ignite deuterium). It is like with a nuclear fusion bomb. Since hydrogen bombs explode because of their density and not only their mass, can this also happen to stars? Or did I miss something? Or is a density to ignite deuterium at lower masses simply impossible? If I'm right, what would be the critical density to ignite deuterium at 1 Jupiter mass?


texts: Infalling observer could never cross Black Hole event horizon?

texts:
I have an interest in cosmology, but I would no way consider myself an expert. It would be great if the more knowledgeable contributors would be able to answer the question I have posed.

The intense gravitational field near a black hole causes time dilation. This means, to an observer close to its event horizon, time  runs more slowly. It can mathematical be expressed as:

Time dilation = $\frac{\delta t_f}{\delta t_0} = \frac{1}{\sqrt{1-\frac{R_s}{R}}}$.

Where $\delta t_f$ is an interval of time a long distance from the black hole, $\delta t_0$ is the time interval a distance $R$ from the black hole’s centre and $R_s$ is the radius of the event horizon.

This formula means that time dilation becomes infinite at the event horizon so to a distant observer a falling object would take an infinite time to reach the event horizon and would never cross it.  However, in the frame of an observer falling into the black hole it should them take a finite time to reach the event horizon. To me the complexity is that all black holes are believed to ‘evapourate’ and eventually disappear in a finite time, by emitting Hawking radiation. Although this an incredibly slow process. For example, a solar mass hole would have a lifetime of $10^{66}$ years (in a time-frame measured a distance from the black hole).

My Question
Assuming Hawking radiation exists and  black hole evaporation occurs, does this mean that an infalling observer would never actually reach the event horizon, because the black hole has finite lifetime and would evaporate before an infalling observer would reach it?

Further Note
Since posing this question I have had one further thought, which may or may be relevant, since my knowledge of GR and quantum theory is pretty sketchy.

The way that the time dilation formula works is that when the observer is a distance of one Planck length ( ~ $10^{-35}$ m ) away from the event horizon of a solar mass black hole (which has radius ~3km), the time dilation would be still only be  ~ $10^{19}$. However, the lifetime of a solar mass black hole is ~$10^{66}$ years. Therefore, even when they are within 1 Planck length of the event horizon, the time dilation near the black hole is not sufficient for the observer to see the black hole evaporate.


texts:
What is the name of the point of intersection of the great circle passing through the celestial north pole P and the Zenith Z, with the celestial equator? (Point T in the figure)
In Italian this point is called "Mezzocielo" (English machine translation), which translates into mid-sky, since it is located halfway between the Zenith and the horizon for an observer in Italy ($\phi \approx 40^{\circ}$).


texts: What determines the surface material of the ALMA and Spitzer telescope?

texts:
The transition between metal dish and mirror happens around hundreds of microns. For example, the shortest wavelength the ALMA(metal dish) can detect is about 0.3mm, while the longest wavelength the Spitzer(optical mirror) can detect is about 160 micrometer.

My questions is what determines the surface material of the ALMA and Spitzer telescope? Could a metal dish observe shorter wavelength than a mirror telescope?


texts: Planets are usually found by observing a star and waiting for the light level to drop when a planet passes in front of it, but what about rogue planets that don't have host stars?


texts: South-eastern sky above the tree line just now, NE Cape Cod, I saw a very bright, twinkling "star"

texts:
I live in New England, Cape Cod to be exact.

I saw a very bright, twinkling object. I watched it for several minutes and it never moved. It was in the east just above the tree line, what might it be?


texts: Hypothetically, if Dyson spheres were already present, how could we observe them? My layman understanding of the reason we believe dark matter exists is that our current estimates of the total mass of the universe is very different from the observed stars. But what if a significant proportion of those stars were covered? My obvious hypothesis is that this would explain the missing mass of the universe, and solve both the question of dark matter and the existence of aliens.


texts: Could we see planets around active Stellar Mass Black Hole?

texts:
This question is a followup of Is there any theoretical or empirical research about planetary systems with a black hole in their center?;

Since it is possible to detect planets around stars with the change of light they cause, I would think it should also be possible to detect them when they orbit around an active Stellar Mass BH.

From what I (can) understand from the first answer, there is no known case of planets orbiting around Stellar Mass BH. What could be preventing us to observe it?

Is there too few active Stellar Mass BH close enough for us to have a chance the system's plan is well aligned with us?
Do we only see Stellar Mass BH when they have a stellar companion feeding them, and this discards to possibility for close-by planets?
Or is it because the light we can see from them is too fast-changing for a wobble or transition to be observable with today telescopes?


texts: Are there any reliable optical measurements of the radius of a neutron star?

texts: I know that there is plenty of theory to predict the size of a neutron star:  my question is whether or not there are any reliable size estimates based on observation. Please note that, to be reliable, such an observation would need to have also a good estimate for the neutron star's distance from us, which must necessarily be reckoned from some independent source, i.e., it must not be reckoned from the apparent size of the object, or from theory, otherwise one is guilty of circular reasoning!


texts: I'm an architecture student and my graduation project will consist of building a research center for outer space, my question is whether or not height would affect sky-glow or not ? Thanks in advance, any help is appreciated.


texts: Why did supernova 1987A's "String of 'Cosmic Pearls'" appear to be so lumpy 20 years later? Is it still?

texts:
NASA's A String of 'Cosmic Pearls' surrounds an exploding star is beautiful and the page says:

This image shows the entire region around the supernova. The most prominent feature in the image is a ring with dozens of bright spots. A shock wave of material unleashed by the stellar blast is slamming into regions along the ring's inner regions, heating them up, and causing them to glow. The ring, about a light-year across, was probably shed by the star about 20,000 years before it exploded.

Did the ring break up into "pearls" or lumps due to some kind of instability? Why so lumpy? The image was taken 20 years after the supernova was first observed, does the ring still look like this?

Also see https://en.wikipedia.org/wiki/File:HST_SN_1987A_20th_anniversary.jpg


texts: Does the orbital variation in planetary gravity affect the Sun's corona

texts:
Dimitris (see below) argues that the syzygies of the Earth and Venus and those of Mercury, Earth and Jupiter distort the Sun's corona, which in some way affects climate on the scale of hundreds of years.  Is there any evidence to support the idea that the planets do have such an effect on the sun, other than the correlation noted by Dimitris?

Planetary orbits’ effect to the Northern Hemisphere climate, from solar corona
formation to the Earth climate.

Poulos Dimitris

Abstract
The four planets that influence the most the solar surface through tidal forcing seem to affect the Earth climate. A simple two cosine model with periods 251 years, of the seasonality of the Earth – Venus syzygies, and 265.4 years, of the combined syzygies of Jupiter and Mercury with Earth when Earth is in synod with Venus, fits well the Northern Hemisphere temperatures of the last 1000 years as reconstructed by Jones et al (1998). Later reconstructions that give too much emphasis on multy-centenial variation are due to increased error. The physical mechanism proposed is that planetary gravitational forces derange the Solar Corona that in turn deranges the planetary geomagnetic field causing
temperature variations.

http://www.itia.ntua.gr/getfile/1486/1/documents/PoulosPaper.pdf


texts: Is it relevant to ask the limits of space?

texts:
Yesterday I watched "The Orville" episode 4 and there was interesting moment where a kid and old man saw the space for the first time.

  Kid: What is this?

  Capitan: Kid, that is space.

  Old man: I used to imagine how it would be. It was nothing like this.
  How far does it extend?

  Kelly: No one knows. Probably to infinity.

In real life we also occasionally ask ourselves if space is finite. And I think it's not a relevant question. We tend to adjust everything to Earth standars where everything is limited and finite. It's very hard for us to accept that something can be infinite, that we even think universe is inside of something and has a material "shell" or a wall. But even if there was wall it should be infinite. So in reality our "wall" is probably empty space or hard vacuum or infinite universes that we will never see because they are infinitely far away, well maybe most of them.

So it's that simple to accept that space is infinite, that we should not even ask that question, since it's so irrelevant and space can only be infinite.

Sorry for my english and the question, since you probably know this and much more, I just wanted to write something.


texts: Would studying massive stars that are experiencing time dilation due to proximity to a black hole be useful?

texts:
I saw this question (Could a star closely orbit a black hole long enough for the star to have lost 0.5B+ years to time dilation?) and came up with an interesting thought. Massive stars live for less than 100 million years, but if it is traveling at relativistic speeds ($\sim 0.1c$) by orbiting an SMBH, we could observe it for slightly longer. Would this be useful, or would it have to be the other way around?
NB: By "other way around," I mean an observer moving at $\sim 0.5c$ or so watching stars age and die.


texts:
What is the value of Saturn's "tropical year" (as opposed to its "sidereal year").

I am seeing it listed as 10 days shorter (10,747 days) than the sidereal year (10,759.22 days).

How could this be if the precession of the axis (equinox) is so slow (~1.86M years)? Is this an "transient/osculating" value? Frame of reference (e.g. J2000)? Am I completely missing something?

I think I can calculate the tropical year by (close to, this works for Earth):

$P_{tropical} = P_{sidereal} \cdot (1-\frac{P_{sidereal} }{P_{precession}})$

Using NASA planetary facts data on Saturn
(these figures are re-used on many websites):

$10759.22\cdot(1-\frac{10759.22}{1.86\times10^6 \space\times\space 365.256})=10759.05$
Using JPL Horizons data:

$10755.698\cdot(1-\frac{10755.698}{1.86\times10^6 \space\times\space 365.256})=10755.528$

about 0.170 days less.

Also, interestingly, see the instantaneous sidereal year of Saturn from the graph of JPL Horizon's data, using the sun's centre (blue) and solar system barycentre (red). I think the instantaneous tropical year (at epoch) would always be 0.170 days less.

I think this is where the confusion is: texts have used particular values, and further the same text has picked a sidereal from one epoch (or average), and a tropical from another...

Saturn's instantaneous length of sidereal year in days, at epoch.


texts: How are astrochemical reaction rates calculated? (UMIST)

texts: To model the abundance of molecules in space, using chemical reaction networks we require large databases of chemical reaction rates e.g. UMIST database, I was wondering whether these are reaction rates measured in the lab, or somehow inferred from observations of the abundances of related molecules in space, or even directly observed from satellites in space? Thanks


texts: How many satellites does the Milky Way have?

texts: The small and large Magellanic clouds are galaxies that are orbiting our own galaxy. How many such "galactic satellites" do we know of?


texts: Are any galaxies partially in the observable universe?

texts: Are there any galaxies that are partially with the visible universe? It seems unlikely as most of the universe is empty space however our circumference makes it seem likely as well. It would be a rather interesting thing to know about.


texts: Which moon has a scar as if it was torn in half and put back together?

texts:
When I was a kid I had several beautiful encyclopedias. In one of them there was a description of moon orbiting on of the gas giants that had huge ripples all along its surface.

It said in the text that it's plausible to think that the moon was torn apart and then got stuck back together.

Could you remind me which one is it? I would like to read a little more about it.


texts: How to orient ESO's Milky Way Panorama in a 3D model

texts:
I have been trying hard to find the star "Polaris" in the following image from ESO and Serge Brunier:

From peering at other maps and simulation apps, I think it should be in the top-left quarter, relatively close to the middle (vertically), however taking the brightest starts one by one, I cannot piece together the other stars belonging to Ursa Minor (small dipper).

I want to use this image as a background in a 3D model of Earth. The projection is perfect for that purpose, I only need to rotate it in such a way that Polaris lines up with the north pole. Once that is accomplished I can rotate it around the Earth's polar axis to set the right time of day at a certain part of the year (it does not have to be 100% accurate for my purpose, but it should at least look a bit realistic to an astronomer).

One thing that makes this task quite hard with this high resolution image is that stars seem to pop in and out of existence while zooming in and out, probably due to lossy fast re-sampling algorithms used in my image viewers.

If someone with a trained eye could point out the Ursa Minor constellation for me it would save me a ton of time!


texts: Will the JWST be affected by dust at L2 (gegenschein?)

texts:
Gegenschein is a "faint brightening of the night sky" at the anti-solar point.  A naked eye limiting magnitude of about 7.6 might enable an observer to make out gegenschein.

The Wikipedia article on gegenschein suggests that interplanetary dust at the Sun-Earth L2 point might be responsible for gegenschein.

The James Webb Space Telescope will also be at the Sun-Earth L2 point.

Does this imply that the same interplanetary dust that may be responsible for gegenschein will affect the JWST?


texts: How does the surface gravity on Mars vary between the equator and its poles?

texts:
A day on Mars is similar in length to the Earth's day. On Earth, due to rotation, the surface gravity varies from 9.78 m/s² (0.997 g) on the equator to 9.832 m/s² (1.003 g) on  the poles. I wonder how the gravity varies on Mars where the day is of similar length. We hear of different values, such as 3.69, 3.711 or 3.72076 m/s². All values around 0.38 g. But I wonder what is the exact value of the equatorial gravity on Mars (including centrifugal forces), what it's on the midlatitudes and on the poles.

The Phoenix probe landed near the north pole; do we know whether it has a gravimeter and measured the surface gravity on its landing place? I know the Curiosity rover which landed close to the equator measured a gravity of around 3.71 m/s² from which I conclude the 3.711 value is for the equator (0.378 g).

When I calculate the gravity of Mars with (G x m)/r² it doesn't include the centrifugal force of Mars' rotation.


texts: If a white dwarf collides with a giant star, could it create a TZO?

texts:
Thorne–Żytkow objects (TZOs) form from collisions between neutron stars and main sequence or giant stars. Ultimately, the neutron star becomes the "core" of the giant star.

However, could this also happen with white dwarfs, if the combined mass is below the TOV limit? White dwarfs are degenerate stars, just like neutron stars, so would they be able to replace the star core in the same way? Would anything happen to the white dwarf if the combined mass is beyond the Chandrasekhar limit?


texts: A few years or so ago there was a project that distributed small very low cost telescopes (perhaps around US$20). What was the name of (and links to) that project?


texts: Are gravitational waves part of dark energy?

texts:
Do the current models of the universe consider the energy release of gravitational waves?

E.g. iirc, the last black hole merger detected had a gravitational wave energy equal to ~3 solar masses (hope I'm not mistaken in this fact).

Same for other mergers, and supernovae: the energy related to their gravitational waves should sum up to something.


texts: How will the recent appearance of a supernova, the closest in 27 years, help reduce the uncertainties in our measurements of dark energy?

texts:
A supernova has been recently spotted on M82, also known as the cigar galaxy. Being the closest to earth in 27 years and given the advances of technology this article proposes that:

  ...as the Cigar Galaxy has been studied in detail thanks to images from the Hubble Space Telescope. "It is likely that the star that blew up has been directly imaged"...

  If astronomers do find the dying star in past images and are able to make accurate readings of its brightness before and after the explosion, we might be able to reduce the uncertainties in our measurements of dark energy by a factor of two.

How will it help reduce the uncertainty?. Furthermore, how would this help us better understand the way the universe is going to end?.


texts: Why is space-based VLBI scattering sub-structure "Hopefully, a new promising tool to reconstruct the true image of observed background target(s)"?

texts:
@HDE226868 mentioned in the observatory that the GBT@20 – The Celebration; Twenty Years of Innovation and Discovery was going on. There are plenty of YouTube videos and slide sets available as links. One of which is Yuri Kovalev (LPI and MIPT Moscow)'s Space VLBI and the GBT.
Below is a screenshot that mentions

Discovery of the scattering sub-structure

A tool to probe turbulent interstellar medium.
Must be taken into account by high resolution VLBI experiments.
Hopefully, a new promising tool to reconstruct the true image of observed background target. Critical for SgrA*.

and links to (these are my best-guess links)

Gwinn et al. (2014): Discovery of Substructure in the Scatter-Broadened Image of Sgr A*
Popov et al. (2017):  PSR B0329+54: Substructure in the scatter-broadened image discovered with RadioAstron on baselines up to 330,000 km

The caption from the image from Gwinn et al. (2014) reads:

Figure 2. Correlated flux density of Sgr A∗ at λ = 1.3 cm plotted against baseline length. Squares show the sensitive GBT-VLBA baselines, circles VLBA-VLBA baselines. Each point represents a 15-min vector average for one baseline after self-calibration; error bars show statistical ±1σ. The green curve shows the correlated flux density of the average scattering disk for an East-West baseline, as described in Sec. 2. The gray curves show quantiles of the predicted distribution of correlated flux density from substructure, with source and scattering parameters from Bower et al. (2006, 2014b), as described in Sec. 3 below.

Excess power at long baselines implies excess fine spatial structure. From what I understand this is belied to be a result of "turbulence" within the intervening interstellar medium, rather than from the original source being observed, and without being characterized and measured presents a serious limit to the resolution of VLBI, both EHT-scale and space-based, and apparently "Critical for SgrA*."
Question: This all seems to be somewhat analagous to astronomical seeing in that it needs to be understood and even measured before it can be compensated for, in order to obtain diffraction (baseline) limited resolution. But why is it "Hopefully, a new promising tool to reconstruct the true image of observed background target."?


texts: On a lunar map, why is it labelled East on Earth, West on Moon?

texts:
source
Why is east and west reversed for the moon? Why does north and south remain the same?
Please explain like I'm five.


texts: What is a "21cm/mm absorption system" in the context of measuring old photons from quasars?

texts:
This answer links to Wikipedia's Time-variation of fundamental constants which cites Further Evidence for Cosmological Evolution of the Fine Structure Constant (J. K. Webb et al. 2001), the abstract of which says:

  We describe the results of a search for time variability of the fine structure constant, alpha, using absorption systems in the spectra of distant quasars. Three large optical datasets and two 21cm/mm absorption systems provide four independent samples, spanning 23% to 87% of the age of the universe. Each sample yields a smaller alpha in the past and the optical sample shows a 4-sigma deviation: da/a = -0.72 +/- 0.18 x 10^{-5} over the redshift range 0.5 < z < 3.5. We find no systematic effects which can explain our results. The only potentially significant systematic effects push da/a towards positive values, i.e. our results would become more significant were we to correct for them.

The paper discusses the "21cm/mm absorption systems" further but I don't understand yet because I'm simply not familiar with the kind of work described here.

The text does refer to:

  HI 21cm absorption lines can be compared with molecular transitions detected at mm wavelengths to constrain $g_p \alpha^2 $ ($g_p$ is the proton g-factor).

so I am thinking that "system" refers to a system of transitions rather than a piece of observational equipment. Is it possible to describe the basics and/or point to a source that does?


texts: What does the ']' in the spectral line "CIII] 1909 Å" mean?

texts:
The above emission line, as I understand, is a useful probe of early star-forming galaxies.

However, I do not understand what the ']' to the right of the 'CIII' means.

I could not find any online reference that can help me decipher this and other spectral line names (for example what does the double lambda in 'O III] λλ1661' mean. Any good reference is appreciated.


texts:
According to the Extrasolar Planets Encyclopedia, the B-subdwarf Kepler-70 lost its helium envelope and became a blue-white subdwarf roughly 18.4 million years ago.

(sources: archived encyclopedia page, also the paper "A compact system of small planets around a former red giant star")

This has got me curious regarding the lifespan of these stars - 18.4 million years isn't that long in stellar terms!

Although I realise this will vary depending on e.g. mass of the star, is anything known about the length of time a B-subdwarf will remain in the subdwarf state before finally becoming a white dwarf? The information quoted above for Kepler-70 was a lot easier to find than for any other blue-white subdwarves.


texts:
This image depicts our solar neighborhood
Source
The plane defined by the concentric circles, does it have a relationship to any galactic reference points or the ecliptic?


texts: At each stage and during each transition of the Sun, is there a region the Earth could theoretically orbit and support life?

texts:
As I understand it, the Sun will transition from its current phase as a 'yellow dwarf', to a red giant, and finally to a white dwarf.

Even during these phases I understand there are transitional considerations, including the increases in luminosity nearing the end of the main phase, reduced temperatures, various ejections and other things being put into solar system, and even the formation of a nebula during the white dwarf phase.

At each stage, sub-stage, and during each transition of the Sun, is there a region the Earth could theoretically orbit and support life similarly to how it does today?


texts:
I've been presented with this problem:

  Say that Jupiter, with its diameter of 142,000 km, was located where Mars now orbits.  What would be the angular size of (the newly-relocated) Jupiter during a close approach, when its distance would be 79,300,000 km?  Would we be able to see Jupiter as a round object with our unaided eye, or only as a point of light?

The angular size is easy enough to calculate given $ \frac{Angular \space Size}{206,000} = \frac{Linear \space Size}{Distance} $.
So $\frac{Angular \space Size}{206,000} = \frac{142000}{79300000}$ meaning Angular Size = 369 seconds of arc.

However, would the unaided eye be able to see this? Knowing that the eye can detect 1 minute of arc across, surely  it can detect 6.15? Any help appreciated, thanks!


texts: Spinning black hole vs non spinning black hole

texts: Does a spinning black hole have a stronger gravity than a non spinning one.After all, a spinning black hole is stretching space-time more, so does that make a difference is gravity?


texts: Planet X prediction by V. B. Ketkar; how could a search for the 2nd planet (Vishnu) be conducted?

texts:
The Indian Astronomer V. B. Ketkar (1853-1930) predicted TWO trans Neptune planets in 1911 in a letter to Bulletin of Astronomical Society of France in May 1911, page 277. Laplace Laws of satellites of Jupiter were modified and applied by Ketkar for arriving at this results.

The first planet (Brahma) discussed there is subsequently discovered as Pluto. The second planet (Vishnu) has not yet been discovered, and is currently referred to as "Planet X" by Astronomers.

Can one calculate the current location and observe it?


texts:
I see that there are many maps of a single galaxy, the Milky Way, showing the positions of the stars within our galaxy.

I could not find a map of Galaxies, however. Is there such a map? In lieu of that I suppose I could make my own map if I knew celestial coordinates of all the nearby galaxies and the distances to those galaxies. Is there such a data table readily available?


texts: What accounts into calculating the Hubble constant?

texts:
From my understanding, the Hubble constant $H_0$ calculates from observed redshifts $z$ of distant galaxys against their proper distance $D$. The current value appears to be 67.80(77) $\frac{km}{s}Mpc^{-1}$

Calculating the Hubble constant via the redshift, I assume one only wants those velocity contributions due to the expansion of the universe, and not those from the real movement of the galaxies within in cluster (peculiar motion) or so. This, I also read in various online resources (which I can't recall right now).

On the other Hand, if your galaxy cluster is far enough away (eq. ~ 1 Gpc), one can neglect peculiar motion, which is in the order of 1000 $\frac{km}{s}$ (1000 $\frac{km}{s}$ / (1 Gpc $\times$ 67.80(77) $\frac{km}{s}Mpc^{-1}$) $\approx$ 1.4%)

Nonetheless, how would you try to correct for the peculiar motion, or is it really just neglected? Calculating all the gravitational components in each cluster? Another idea could be to assume that the galaxies within the cluster move randomly in respect to each other and the peculiar motion cancels out over the average? Any other possibilities?

Note that this question is partly a copy of another question of mine at physics.stackexchange.com, which wasn't fully answered but commented to ask it here.


texts: Estimating Sun luminosity from first principles

texts: In a lecture, it was claimed that luminosity can be estimated by dividing how much energy there is in the Sun with the typical time it takes for a photon to get out from the centre of the Sun to its surface. I don't see how that argument holds. Why would that give me luminosity?


texts:
While studying photometric redshifts, I came across the ugriz (or u'g'r'i'z) system for classifying magnitudes of galaxies, but I didn't find much information on the internet about how it works.

Can somebody explain that? Do the letters "ugriz" have any special meaning, or is its just a nomenclature?


texts: I have seen a lot of information about moon phase calculation, but need an accurate moon shape calculator/algorithm given the observer's position and time. This implies that the illuminated portion may be rotated, and not only have a right-left orientation as traditionally depicted.


texts: What is supposedly in the center of the Milky Way?

texts: It's big. It's bright. It's white. It's so intense. But what is at the center of the Milky Way?


texts: I may be way off the mark here, but wouldn't it be possible to use kugelblitz (artificial black holes formed from a concentration of radiation rather than matter) to significantly delay heat death? The black hole would reconcentrate diffuse matter and turn it into usable energy. It may not be especially practical due to the limited range of the kugelblitz's gravitational field, but if nothing else, couldn't it be used to create energy by feeding it matter? With its smaller size, it would release the produced energy through Hawking radiation much faster than stellar or supermassive black holes.


texts: Since Proxima Centauri orbits the other two Alpha Centauri stars, will it eventually be farther away from us than they are?

texts:
And, if so, will we have to change its name to Distal Centauri ;-) ?
Granted, its orbital period around the other two is about 550,000 Earth years, so we have some time....


texts: Is there any point on earth where the moon stays below the horizon for an extended period of time?

texts:
When I was teaching a class why the earth has seasons today, I mentioned how the poles of the earth experience months of daylight and darkness. Then one of my students asked whether the moon disappears as well.

Using an orrery and a string, we looked at the orbit of the moon around the earth and tried to find a place where the moon was below the horizon for an entire orbit. In our attempts, we could find no such place, though some students speculated that if you were in the right place at the right time, the moon could make a full orbit while partially obscured by the horizon.

Is there a place on the earth where the moon cannot be seen on for long periods of time, like the sun in northern latitudes? Or does the fact that it orbits around the earth make this impossible?


texts: what happens when we keep going radially away from earth?

texts: Is the generally accepted belief in the science community, that if one sits in a spaceship and move away from Earth where r (distance between earth and my ship) is continuously increasing, can one keep going forever?


texts: In a planetary system close to the galactic core, would it be possible to see the supermassive black hole?

texts:
I understand that radiation on planets near the galaxy’s center make life on these planets nearly impossible, and that one cannot truly “see” a black hole. However, if you could stand on the surface of a planet that orbits a star close to or within the galactic core, could you theoretically look up at the sky and see an absence of light/stars indicating the location of the central supermassive black hole?

Would it be too far away to see, obstructed by debris, or too small to notice?


texts: Are there small voids of 10 - 20 megaparsecs also within, and not only between superclusters?

texts:
I sincerely apologize if this question is stupid, however after all the articles I read so far, I could not confirm this assumption, which I think is actually a fact, but I'm not completely confident.

I'm writing on the topic of proportions of the universe, and in one of the statements I claim the following:

"Just one small void inside a supercluster of galaxies can span across 10 to 20 megaparsecs (32 to 65 million light-years ) in diameter."

Let me state that I understand that a void within a galaxy supercluster may not necessarily be completely surrounded by galaxies of that supercluster, and can extend far out from the supercluster it partially occupies, so to clarify what I mean by my question on a practical example:

If you were to draw a straight line across a map of a galaxy supercluster, can that line start inside a group of galaxies of that supercluster, go across a void of 10 - 20 megaparsecs, and end inside another group of galaxies of that same supercluster?

(I am basing my "small" classification of a cosmic void 10 - 20 megaparsecs in diameter on the Wikipedia article "Void (Astronomy)" https://en.wikipedia.org/wiki/Void_(astronomy) in which it says:

" ...Voids typically have a diameter of 10 to 100 megaparsecs; particularly large voids, defined by the absence of rich superclusters, are sometimes called supervoids. ..."

My only goal is basically to find out whether I would be wrong or not if I state that small voids of 10 - 20 megaparsecs, really actually exist within, and not only between superclusters.

For any eventual help on this topic, I would be sincerely grateful.

Kind regards


texts: Can't pulsars and stars be used for gravitational wave measurement?

texts:
Suppose that we collected photons from a distant star, and they arrive regularly at 15 photons every microsecond onto a CCD, when a gravity wave bends space time, wouldn't the regular 15 photons graph wobble as a result of the gravitational wave?

Why does the LISA observatory have to compare laser timings, if pulsars can be as precise as atomic clocks, can't it just compare a bunch of pulsar signals? A single satellite that records 50 pulsars would move compared the the pulsars and would have a frequency of 50Hz ticks with pulses which can be graphed at kHz frequencies.


texts: I want to know the explanation of Phillips relation, the relation seen in type Ia supernovae(SN Ia with broader light curve is brighter). More Ni-56 implies a larger luminosity at peak but what about broader light curve?


texts: Can a solar flare destroy every electronic item on earth?

texts: I was reading a book about the sun which suggested that a solar flare can destroy every electronic item on Earth. Is this really true and if so, how would it happen? Could we do anything to to prevent it?


texts: How often does the supermassive black hole in the Milky Way become a quasar?

texts: My current interest is black holes and quasars. I was wondering how often they erupt--specifically ours? Would it be blindingly bright on Earth? Finally, would there be any effects that would alter our life on Earth that we know of?


texts: At what rate is the observable universe expanding?

texts:
The observable universe is $4.4 \times 10^{26} m $ in every direction. I told this to my daughter and then added, "And guess what? It's a little bigger today than it was yesterday."

"Wow!"

But by how much? My first thought would be: one light-day, in each direction. But that can't be right. The boundary is receding faster than the speed of light.

Yesterday, a lonely photon ended its 13.8-billion-year journey with the collapse of its wave function (or whatever, let's not start on this) on a rock orbiting a smallish star toward the edge of a spiral galaxy in a relatively sparse region of the Universe.

Today, yet another photon met a similar fate.

How much further away was the 2nd photon when it began its trek?


texts: How do we know that Singularity is not an object with reasonable radius?

texts:
So for example: a Neutron star has a radius of 10km and 2 solar masses.

A singularity can have a radius of 1m and 10 solar masses, with a 1km event horizon radius. Basically it will just be a more compact object than a neutron star. How come we are so sure that singularity is an infinitely small point instead?


texts: In India, what is the best time to view the geminid meteor shower in 2013

texts: As of the time of writing this question, the meteors should have appeared, but so far I haven't seen any. What time would be the best to view them?


texts: Could a distant black hole (detected due to its mass) be in reality a Dyson sphere?

texts:
When the black hole is only detected due to its gravitational effects on other stars and their light, could it be possible that it is in reality a working Dyson sphere?

It would still have at least the mass of the star but also blocks all the emitting light.

I got the idea from watching this Kurzgesagt video about Dyson spheres


texts: How can I calculate the time between sunset and actual darkness?

texts:
I would like to know the time it takes to go from sunset to dark based on GPS location.

In addition, ideally I would also like to have a formula that determines the percentage of sunlight at each minute past sunset. For example, after 15 minutes, there is only 50% sunlight left.


texts: the length of second, minute or hour, what defines the time of exo planetary bodies

texts:
we measure time based on earth and earth mass\gravity plays a role in the measurement ? so how and what would be time if not earth time on other exo planets since they will be having different mass/gravity and probably more variables affecting time measurement.

I can say Gliesa has a day of 30 hrs(assumed) ,

So will the length of second, minute and hour, likeewise day or month will fall under its own time system?


texts: Could supermassive black holes form in dwarf galaxies?

texts: Supermassive black holes often exist in large galaxies (e.g. the Milky Way, Andromeda). They make up a fair portion of the galaxy's mass. Is it possible for a supermassive black hole to form in a smaller galaxy - a dwarf galaxy (i.e. the Sagittarius Dwarf)?


texts: Is it possible to use the stars to determine the passage of time?

texts:
I'm writing a science fiction short story which involves a group of people being in suspended animation for a very long period of time, on the order of thousands of years.

My question is, would an astronomer on waking be able to determine that a significant period of time has passed purely by making observations in the night sky? How accurate would they be?

For the purposes of the story, I'm more interested in what an astronomer could observe unaided or with a primitive telescope of the sort Galileo might have had early in his career (around 3X magnification).

Thanks!


texts: Scientific possibility of stationary moons for a writer of fiction

texts:
I tried to do some research to find an answer for this question before posting this but didn't find anything that I could sink my teeth into.

I was wondering:

if it would be possible for a planet to have a stationary orbit Moon, so only one side of the planet would see the moon ever
what that would do to the oceans and tides of the continent that is on the opposite side of the planet.

Conversely how would oceans and tides be different without the moon?

For my question here I'd like to ask about only the planetary science aspects. For the speculative and hypothetical aspects I can ask a separate question in Worldbuilding SE.


texts: Could a binary system of two planets with oceans reflect each other?

texts:
If there were two earth-like planets in a binary system and one of them had landmasses, and the other one had oceans covering a significant amount of its surface, could you see the continents of the first planet reflected onto the oceans of the second one?

If so, could this also work with oceanic moons?


texts: Understanding The Turnover Point of Angular Diameter Distance

texts:
I am trying to get a better understanding of cosmological distances, in particular the angular diameter distance which I have also seen referred to as angular size distance.

What I am looking for is a more detailed explanation of why the angular diameter distance "turns over" in the following graph.

Wikipedia states in their article on angular diameter distance that objects beyond a certain redshift have a smaller angular diameter and as such appear larger on the sky.

I have searched around and have been unable to find a complete explanation of this behaviour and have only found brief answers which state the expansion of the universe or the size of the universe.

I would like if possible for a more detailed explanation as well as the physical significance if any at all of what it means for an object to be at this redshift when they emitted light we are now observing.

Thank you for any feedback or and discussion or suggestions where to look. I would also like to include as it may be of some help that I am currently in my final year of undergraduate studies in physics so please do not hold back any details or technicalities!


texts: What made the recent supermoon so super?

texts:
I understand that a "regular supermoon" is a full moon when the moon is at perigee (closest to earth in its orbit).

They seem to happen often, here's a post from one last year. But I've seen news articles saying this is the 'superest' super moon since 1948 and we won't see another super moon this bright until 2034.

What makes the recent super moon so bright?


texts: What decided how the Kepler space telescope was pointed?

texts:
I suppose that the Kepler field was well chosen. It is rich in stars and has minimal background light since it is pointing out from the disc.

But other things could have been considered. For example, a field which contains the most stars which are near enough to be followed up by ground based telescopes. And why wasn't Kepler designed to change its pointing so it could cover several fields, such as all of the above? Funny enough, it does so now as K2 after two of its reaction wheels have broken, so studying only one field doesn't seem to be a technical issue, but something which astronomers prefer. Why?


texts: I am new to stargazing, so I am unsure of what I am doing. I have an Inspire 80az telescope and 20mm and 10mm lens. I can view the moon really well but would like to view more planets, so I want to know what lens I would need to see others.


texts: Why does matter stay collapsed in the core, following a supernova explosion?

texts:
Following a supernova explosion a star may turn into a white dwarf, neutron star, black hole, or just a stellar dust & gas leftover.

Excluding the latter case, why and how does the star's core matter stays collapsed, after such an event where matter is burst and scattered in space?


texts: Do all the stars visible to the naked eye belong to Milky Way?

texts:
It is surprising that even late in 1920, people do not know there are galaxies besides the Milky Way.

Does this mean that all the stars visible to the naked eye are near to us?


texts: Is the exact duration of the shortest day of a year (at the winter solstice in the northern hemisphere) always the same, for given latitude and longitude, or does it fluctuate? If it does, what is the range?


texts: How open is China in sharing data from the Chang'e 4 mission?

texts:
It's really good to see that man (well, machine) has gone back to the moon again.

Chang'e 4 is studying potential mantle material from the lunar depths and will be conducting radio astronomy in frequencies that simply aren't possible from Earth.

There's a huge amount to be learned that will fill some gaps in our knowledge of the early Moon, Earth, and the Solar system as well as the wider galaxy and beyond.

Will this information be open for the greater scientific community, or will we have to rely on the Chinese scientists to collate and publish the data as and when they can do?


texts: Is the universe macroscopically transparent to CMB? Is the fraction intercepted by stars and dust so tiny that it doesn't have a correction factor?

texts:
Background
The question Why do some electromagnetic waves continue travelling while others disappear? is interesting, and in addition to the answer there I started to write:

This is a supplementary answer to @ConnorGarcia's answer using the example in the question. It's not very accurate but it does address part of the question:

Cosmic background radiation emitted when the Universe was very young still exists. But my wifi signal seems to disappear a short distance from my apartment. Why?

Imagine walking around in the middle of a city with a receiver that can pull in 900 MHz to 5 GHz. It will tell you that you are in a "soup" of radio photons; no matter where you go inside the city you'll...

and then realized I didn't know what I was talking about.
Question
So instead I'd like to ask about the following:
I think that CMB photons are believed to be "pristine photons" from soon after the big bang rather than absorbed and re-emitted and thermalized, and I seem to remember recently reading an answer here or in Physics SE saying that a measured CMB spectra (in any given direction) is fit with a Planck distribution with unity emissivity; there was no "fudge" or scaling factor in front for the absolute spectral radiance measured.
But now I can't find that answer anywhere!
Together those suggest to me that macroscopically the universe is essentially transparent to CMB, and the fraction intercepted by stars and dust between us and it is so tiny that it doesn't require a correction factor.
Question:  Have I got this right?


texts:
First time astronomer here...  (pardon any less than accurate terminology)
My son received a secondhand telescope, a Celestron Powerseeker 114AZ.
Last night I calibrated the Starpointer, and was able to see Jupiter and it’s moons as well as Saturn and it’s rings.
However, after I position the laser in the Starpointer guide, the tube slightly sinks into a different rest position. To accommodate this, I ended up having to predict this movement by setting the laser guide a little higher than the target and let it sink into resting position. I also notice that the optical tube has a little bit of vertical wiggle on the azimuth bracket, which makes the altitude adjuster less accurate. The top hinge of the tripod legs seem a little wiggly too.
Is this normal movement or should I find a way to tweak, adjust, and/or tighten the parts of or the entire assembly?


texts: Do celestial objects need to be big to have liquid water on their surfaces?

texts:
I mean no asteroid, planetoid that I am aware of has water on its surface. It is way more common to see ice in it. So I figured that the size of the celestial body has something to do with the cycle of water. Does this theory hold true? Do exoplanets with water have to be similar size than Earth? To have water does it has to be a rocky planet?
I mean some moons have water inside their crust (I think). Some planets seem to have had water before, like Mars.


texts: How to chose a comfortable 2 inch eyepiece for a Skywatcher Dobsonian 150 mm f/8 reflector?

texts:
Totally newbie here. I got the Skywatcher Dobsonian 6 inch (150 mm) f/8 Newtonian reflector. I had a brief chance to test it yesterday when the skies here in Melbourne cleared a little bit. I was able to see the Moon that was not that high in the sky.

With the telescope, it came with two 1.25" eyepieces; 10 mm and 25 mm, and a 2" adapter.

I found out the best view with the 25 mm although it wasn't that clear. I still have to check the mirrors alignment and collimate.

However, I found a bit uncomfortable to use it though. It was hard to keep the entire view clear. If I moved slightly I couldn't see the entire view.

After reading some articles, it seem that a 2" eyepiece offer a more comfortable view experience.

I'd like to ask; how can I choose a 2" eyepiece that would be more comfortable for observing the Moon and planets? What should I look for in terms of types or specifications?


texts: Where can I find a good solar irradiance chart for the infrared?

texts:
I'm trying to understand the results of the ExoMars Trace Gas Orbiter vs the Mars Express in terms of their abilities to find methane on the surface of Mars. The solution that I'm coming to is that it might be a result of the Sun's spectra masking what is actually there. Is there a place that has a complete infrared spectra of the Sun, focusing in on the region around 3-4 um? Specifically, I'm interested in wave numbers ($cm^{-1}$) 3000-3100 or so, something with a resolution of 1 wave number. Some things that would help would be anything along the following lines, so long as they apply to the true spectra, and not the one as seen from Earth.

A generally accepted solar spectra (NASA, ESA, NIST, or some other body)
A time varying solar spectra.

Thanks!


texts: Can circumstances exist such that as seen from some solar system all stars are obscured?

texts:
I am mostly active on Physics Stack Exchange but a question occurred to me that I think is best submitted here.
This question is about a thought experiment: whether the circumstances of the intended thought experiment are physically possible. For this thought experiment it's not necessary for the circumstances to be physically possible, but it would be nice bonus.

In the history of astronomy the fact that the fixed stars were available as reference of celestial motion was hugely helpful.
For example: my understanding is that Kepler's approach to finding the shape of the orbit of Mars was as follows. From astronomical records extending over a long time the period of the orbit of Mars can be reconstructed. Kepler would take pairs of Mars position observations, spaced one Mars year apart. These pairs of Mars positions were from two different Earth positions, allowing Kepler to triangulate.
The intended thought experiment:
A solar system in a region of its galaxy with such a density of interstellar gas that all the stars are obscured. Without the fixed stars as reference of celestial motion the Kepler problem would be much harder.
(The purpose of this thought experiment is to argue that while it would have been harder to obtain the inverse square law of gravity, it would not be impossible. This can then serve as demonstration as to what theory of motion is.)

Some of my guesses:
My understanding is that all parts of the galaxy can be observed, it's just that for some parts the observed luminosity of the stars is diminished because the light coming from those stars traveled through regions with interstellar gas. That would make it very unlikely that interstellar gas can be dense enough to obscure absolutely all stars.
That brings me to guessing that this thought experiment would require rather contrived circumstances. For instance, maybe it would require a star to be out in intergalactic space. That would mean that star has had the misfortune, in its distant past, of being ejected from its originating galaxy.

[Later edit]
Assuming the planet is rotating and that the axis has an inclination with respect to the orbital plane:
Once the astronomers make the transition to a heliocentric model:
The astromers have the measurement of duration of the solar day, and from the fact that the duration of the year is constant over time they can, it seems to me, infer the ratio of the solar day to the intrinsic day.
The astronomers can opt to use the line of intersection between the plane of the equator and the planet's orbital plane as a reference of zero rotation rate. (Of course, a rotating planet that has an inclination will undergo gyroscopic precession. However, the precession of the equinoxes is slow enough that on the assumption that the reference is stationary highly fertile science will be possible.)
It seems to me that once such a reference system is in place a Kepler has the opportunity to find Kepler's laws of planetary motion.
The purpose of the thought experiment is to demonstrate this: the very motion of the planets of a solar system is sufficient to establish a reference of motion. I submit that the conservation of the angular momentum of a rotating planet is sufficient to provide a solar system wide reference of orientation. Over time astronomers can iterate to an ever more accurate reference of motion.
Summerizing:
Motion itself is sufficient to provide reference of motion.


texts: Are any Pluto-sized objects remaining to be discovered in the Kuiper Belt?

texts: An object approximately the same size as Pluto, Eris, was discovered only 8 years ago (in 2005). Are there any Pluto-sized objects remaining to be discovered, and if so, how far away from the Sun would they have to be to not have been detected already?


texts: What's the theoretical maximum density of a galaxy?

texts:
In watching this Hubble video on Omega Centari it got me thinking about how dense a galaxy can get. Do we know how dense a galaxy can get?

Are there any good resources on reading more about very dense galaxies?


texts: If one were to drop, say, a pebble, from a sufficient distance, into a black hole. The pebble would continue to accelerate until it were just short of the speed of light. In the next moment it's speed should exceed the speed of  light. Why not?


texts: Definition of space - to cope with "space expansion"

texts:
I've searched quite a lot but found no satisfying definition, so please:

How do we actually define space when talking of "space expansion"? Isn't it just a metric, a dimension we measure, instead of an entity? How to define the "fabric" that is increasing? I can only understand it in one of these 2 ways:

1) The "distance unit" (size of the meter) is increasing, meaning light is also "dragged" with the increase. Because we define 1 meter as the difference in spatial position between 2 objects which light takes 1⁄299792458 seconds to cover. So, it means in this case, light is stretched, to cover the same distance in the same time. But also, each wavefront of the light beam covers the meter in the same time as before, it's still 1 meter for all of them. So it means the beam on the whole, even if longer, also travels faster to exactly compensate for the elongation ("longer" and "faster" are relative to the "old" meter). So I don't see any explanation for redshift appearing. Also, if the meter would be increasing, I can't understand how they say that distant regions of space are "growing appart" "faster than light" - since the distance unit is defined in relation to light in the fist place.

2) The number of "distance units" (number of meters) is increasing - which seems to mean actually movement instead of metric expansion. The objects have more and more meters between them, meaning light takes more and more time to travel between them (because we define one meter in relation to light). But it seems not true, because they say big regions of space expand faster than light - and objects can't actually move (rate of meters increasing) apart faster than light, do they? Or are there maybe 2 ways of moving - the one which is relative to the mystery dark matter/energy filling the "void" and so it's a moving bound by light speed, then the other kind of moving, which includes the effects of dark energy, and is unbound by light speed?

Or, to rephrase without the level of abstraction of the meter:

Is the space expansion dragging light?

1) If yes, why do we have cosmological redshift and why do we have space expanding faster than light?

2) If no, it’s interesting to see light having a special treatment. Also, this case seems very close to “movement”.


texts: Please Guide me to buy my first Telescope

texts: I am a beginner and I've planned to buy a telescope(Newtonian reflector)for viewing planets and deep sky objects. As far as I know the most important factor to get a brighter view is that the telescope need to have a good aperture. I need a telescope that would be good both for viewing planets and deep sky objects.So my question is here as a beginner how much aperture is necessary to get amazed with the space. And I don't know much regarding telescope so please let me know the other factors for getting good view of celestial objects.


texts:
I would like to plot a butterfly diagram. I've been looking through the SOHO archive for a table that would indicate the date and position of each sunspot, but to no avail. The closest I got was to use the archived images to create the table myself, but that would be extremely fastidious.

Is there a place where I might find this kind of data?


texts:
how to connect two eyepieces together with the x3 Barlow lens

I have 10mm,25mm, and x3 Barlow. Is there any way to connect them together to archive the maximum magnification of 105mm?


texts: I'd like to become an astronomer. What experience do I need?

texts: I think I'd like to become an astronomer. What degrees and training/experience do I need to become one? I've heard I should take science and mathematics, but I don't know which specific courses I should take and which degrees I would need to become an astronomer.


texts: How to calculate the point on the globe with maximum magnitude with given Besselian elements and given time for a partial solar eclipse?

texts:
I am currently working on improvement of an Android app that calculates solar eclipses. For many days now I have not been able to solve a mathematical problem concerning solar eclipses, so I decided to try to ask for help.
I am working with the book "Elements of solar eclipses 1951-2200" by Jean Meeus. Calculating the central line of an eclipse from given Besselian elements already works good, so I am able to calculate the path of the central line of total and annular eclipses and the curves of equal magnitude.
My problem now is that Meeus only describes the calculation of the central line, so for a given time, that means for given Besselian elements I can calculate the corresponding point of the central line on the globe. Using these algorithms I can not calculate the line of maximum magnitude for partial phases of total and annular eclipses or for partial eclipses. So my question is:
How can I calculate latitude and longitude of the point on the globe with maximum magnitude with given Besselian elements for a time when eclipse is partial, that means, when no central line exists. As far as I understand the problem, this should be the point on the globe that is nearest to the shadow axis, which in the case of a partial eclipse or partial phase of an eclipse does not intersect the globe.
I would very much appreciate any help.


texts: Why do Jovian moons fade when transiting Jupiter's shadow?

texts:
When reading an article on using Jovian moons to calculate longitude, I came across this passage:

  Periodically, Jupiter eclipses each of the four large moons as they pass into the planet's mammoth shadow. In a telescope, a moon's brightness takes several minutes to fade to black as it enters the shadow. Reappearances are just as gradual.

How can the moons take this long to fade in and out? Is it simply the shadow creeping over the surface of the moons? What does this look like?


texts: Any new info about solar flares hitting the Moon added by the LADEE mission?

texts:
This article from 2011 mentions simulations about the sputtering effect caused by a solar flare hitting the Moon:

  "We found that when this massive cloud of plasma strikes the moon, it
  acts like a sandblaster and easily removes volatile material from the
  surface," said William Farrell, DREAM team lead at NASA Goddard. "The
  model predicts 100 to 200 tons of lunar material – the equivalent of
  10 dump truck loads – could be stripped off the lunar surface during
  the typical 2-day passage of a CME."

The researchers said they were waiting for LADEE to confirm these simulations and add new data, but I didn't find any conclusions on this subject on the mission website. I was interested if this event can create a significant static electricity difference between the 2 sides of the Moon, causing some sort of lightnings on the edges or even a mini-version of a dust storm.


texts: Is it possible to find what crater the Mars meteorites come from?

texts:
A few dozen meteorites have been determined to come from Mars. These meteorites likely formed from three separate events.
Each one of these events must have created a big enough crater that rocks would escape Mars' gravitational field. This restricts the number of craters to look at.
The composition of the meteorites can also give clues. For instance, the basalts on the floor of Gusev crater are somewhat more mafic than the meteorites, so they don't come from Gusev.
Using crater size, rock composition,... would it be possible to determine which craters the Mars meteorites come from?


texts:
I have recently learnt about Hubble's Law and this is my understanding of it:
Galaxies move away from us at a rate that is proportional to their distance from us. So, considering an arbitrary unit of distance d, if a galaxy at a distance d from us is moving away from us with a velocity v, then a galaxy at a distance 2d is moving away from us with a velocity 2v. This proves that the universe is expanding.
But the part that I can't quite visualize is the fact that it doesn't matter where you are in the universe. Regardless at what point in space we are standing, it will look like the galaxies are moving away from us. How is this possible? Are the galaxies moving outward? Should I visualize this as the galaxies being on the surface of a sphere whose radius is increasing?
How do I gain intuition about this and better visualize this?


texts: Is there any theory suggesting the expanding universe may be coupled to another shrinking universe

texts: Our Universe is supposed to be expanding. I was curious if there was any theory suggesting the expanding universe may be coupled to another shrinking universe?


texts: How far from the Sun can an Earth TCO (temporarily captured orbiter) go after leaving?

texts:
Objects which are temporarily captured into an Earth-Moon system orbit (temporarily captured orbiters or TCOs) by definition will leave this orbit. While the term suggest an Earth orbit, it's really an n-body orbit involving the gravitational effects of the Moon and Sun as well.

They are also sometimes referred to as "mini-moons".

Upon exiting, can TCOs go as far out as the asteroid belt, or do they then just orbit the Sun in a heliocentric orbit very similar to the Earth's 1 AU?

Has anyone tracked or calculated the orbit of a known TCO to this extent?

As a side question (it can be split this off separately if necessary) have any of these been larger than say 10 meters in diameter?


texts: Spectroscopy question about a Paschen series absorption

texts:
I had this question on a test and got it wrong but I can't understand the explanation.

I don't get why we receive a Paschen series absorption. Could anyone explain?


texts:
I have imported my fits table, hdul0, into python. I'm using astropy.table. I opened it with hdul0 = fits.open('filename').

hdul0data = Table(hdul0[1].data)

I have then made a scatter plot of two columns in the table, HDA and OII. I have made a cut-out region on the plot (a short vertical line and then an upwards sloping line from there), and I would like to select the points under that line, like slice them from the original fits table and move the slice into another fits table that I then save. This is what I have done, but it is not working.

x = hdul0data['HDA']
y = hdul0data['OII']
x1 = [2.5, 2.5] #the x values of the vertical line cut
y1 = [0.0, 2.5] #the y values of the vertical line cut
x2 = [2.5,16.0] #the x values of the sloping cut
y2 = [2.5, 16.5] #the y values of the sloping cut

slopy = np.interp(x, x2, y2)
myslice = np.where((x > 2.5) & (y < slopy))[0]

The error message I get is "RuntimeWarning: invalid value encountered in greater" (and the same for less).

Can anyone help? How do I slice my table and get the selected points into a new fits file? I think maybe part of the issue is that some of the data points in the HDA and OII columns are NaN, but I don't know how to get rid off it. I read I could use masked, but then I read it might not work with numpy.

EDIT:

Sample values of x and y:

x = [1.0, 6.2, 10.4, NaN, 2.8, NaN, 7.0]
y = [10.3, NaN, 18.0, 22.9, 2.5, NaN, 31.8]


texts: How can gravitational waves escape from the gravity of black hole?

texts: I know that even light can't escape from a black hole's gravity and the velocity of light and gravitational waves are the same. How can only gravitational waves escape from its gravity?